Filtration of molten cast metals has been demonstrated to be an effective method of improving overall casting quality. Filtration enhances the mechanical and physical properties of castings by removing inclusions from the molten metal before they can enter the mold cavity.
There are numerous applications in foundry practice where quantities of molten metal are poured repetitiously into holding furnaces for replenishment of metal used, into die casters for each shot and in pigging operations. Those foundry operations that require repetitive or automatic pouring of molten metals, especially aluminum, into casting machines such as die-casters and permanent mold casting operations cannot easily utilize filters in their gating systems. Therefore the metal is usually filtered in a holding furnace dip-well prior to being poured into the machine. These operations typically utilize a robot to dip into the furnace dip-well and then pour the metal into the casting machine. Aluminum is usually the metal cast in these operations, and even though it is filtered in the holding furnace it will form dross or oxides immediately upon exposure to the air as it is dipped out of the dip-well. Being able to filter the metal as it is being poured into a casting machine would be a great advantage. Most of these operations are also high production. For example, die-casting machines typically make 30 to 60 shots per hour. These operations would benefit greatly by having a new filter automatically positioned for filtration of each shot or pour.
There are also many operations, especially in foundries, where batch quantities of molten metal are treated with master alloys, ferroalloys and inoculants that create reaction products such as oxides, sulfides and slags. These impurities cause many problems in holding furnaces such as slag build-up on furnace walls and pouring nozzle clogging, which result in costly down time and excessive maintenance.
In highly automated ductile iron foundries high production molding machines are used that make a mold every 10 to 15 seconds. The holding furnaces that feed metal to these molding lines are typically replenished with 1200 to 3000 pounds of ductile iron every 10 to 30 minutes. Batch treated ductile iron is also transferred from the treatment ladle into smaller pouring ladles. Filtering the metal at this stage would result in much less maintenance of the pouring ladles plus provide cleaner metal to the mold line.
Ceramic filters are extensively used in the foundry industry to improve casting cleanliness and to reduce the number of castings that must be scrapped due to the presence of unwanted impurities. Such ceramic filters are typically incorporated into the gating system in order to remove slag, dross, and other particles from the metal stream before the metal enters the mold cavity. The most common types of ceramic filters comprise hard-fired cellular ceramic structures and rigid reticulated ceramic foam. These filters are relatively thick and, due to their massive structure, tend to chill the first molten metal that reaches the filter, requiring relatively coarse openings to ensure reliable passage of the molten metal. Cellular extruded ceramic filters therefore rely on the formation of a filter cake on the upstream side to remove smaller inclusions that would otherwise pass through the openings in the filter. Ceramic foam filters, while providing a more tortuous path for the molten metal than cellular filters, also exhibit the formation of a filter cake which can become the controlling factor for the size of the inclusions that the filter will remove.
Recently, refractory cloths made from materials such as fiberglass and silica have been used as metal filtration media. The most common refractory cloth for filtering high temperature ferrous metals comprises woven silica fibers. U.S. Pat. No. 5,124,040 to Hitchings, which is hereby incorporated by reference, discloses a silica cloth filter having a carbonaceous coating that produces improved filtering characteristics.
Conventional molten metal filters are typically only able to be used once because of the impurities that they collect and because the molten metal solidifies around them after the pouring operation, particularly when they are used in mold gating systems. Prior art hard-fired ceramic filters and ceramic cloth filters are usually placed by hand before each pouring operation, adding significantly to processing costs. Conventional molten metal filtering apparatuses incorporating ceramic filters are disclosed in U.S. Pat. Nos. 5,202,081 to Lake et al., U.S. Pat. No. 4,990,059 to James, and U.S. Pat. No. 4,159,104 to Dantzig et al.
At the present time there is no commercially available filtration equipment to automatically filter significant quantities of metal with automatic replacement of the filter media, particularly as the metal flows into a holding furnace or pouring ladle. The present invention has been developed in view of the foregoing and to overcome other deficiencies of the prior art.